The present invention relates to an auto-master pedestal control circuit which controls the position of pedestal to be accordance with the dimmest point on an actual screen at the processing of a video signal generated by a video camera.
In a conventional method, for the control of an auto-black balance, an applied video signal has been controlled to be accorded with a pedestal level which is predetermined in a constant level (7.5% for the U.S.A., 5.0% for Japan and 7.5% for Korea) on conditions with a closed lens cap mounted on a video camera. In such method, there is no reference signal to be distinguished for each scene.
However, since the pedestal level is established only with the lens cap closed and each object has different intensity of radiation, it is unreasonable for the fixed pedestal level to be applied to actual photographing for distinctness. The pedestal level established without a signal outside becomes a fixed constant. With this fixed constant of pedestal level it is impossible for the fixed pedestal level to be corresponded to the dimmest points in each respective scene, and it is impossible to obtain respective distinct screen pertaining to respective scene.
Therefore, in the electronic field processing system of a video camera equipped in the relay broadcasting car or studio in which long distance electrical transmission is processed, broadcasting engineer has been manually controlling the pedestal level with directly inspecting the conditions of the screen took by the video camera and through other instruments.
For the video cameras for the electronic news gathering system and general purposes, it is hard for the dimmest points to be accorded with the pedestal position as each dimmest point for each scene is different.